Depending on type of plec most likely to be a bristlenose imo then it will need 25+ gallon although there are some smaller specie pleco's you may have. Neons, guppies, danios all need to be in groups and should be fine with shrimp. I believe razers betta is quite peaceful so he should be fine with adult shrimp if they are given adequate hiding spaces. Neon tetras are known fin nippers so the betta may be attacked by them, the same with guppies attacking him and if the guppies are fancy variety the large colourful tail may get attacked by the betta.
Razer you really need to research and stop impulse buying!